One way it could be done would be to have Frodo start reading from the Red Book of Westmarch – at first we hear Frodo's voice, reciting Bilbo's story in the first person. Then the scene fades to '60 years ago', and Bilbo's voice takes over from Frodo's. This voice could be provided by either Martin Freeman or Ian Holm (I would prefer the latter).

Also, I think 'an extreme flash-forward' is a very complicated way of looking at it. It sound to me more like The Hobbit actually opens towards the end of The Return of the King, and that Bilbo's entire story is presented as an enormous flashback.